Motors are like a good dog. They will try to do what is asked of them, will not question when asked to do something they cannot, will forgive you for minor errors and need regular maintenance. However, like dogs, they cannot read or talk. So, it is up to users to make sure everything is working properly, to ensure that they can have a long, happy life. The nameplate is provided to help users take care of the motor and provide it with the things it needs.
What Is a Nameplate Rating?
According to National Electrical Manufacturers Association (NEMA) motors and generators (MG)-1, “A permanent marking of nameplate information shall appear on each machine, displayed in a readily visible location on the machine enclosure.” The information on a nameplate will vary depending on the type of motor, but most will have an output power (in horsepower (hp) or kilowatts [kW]), voltage, current, rotations per minute (rpm), frequency, service factor, etc., under rated conditions. There can be additional data on the nameplate regarding temperature ratings, bearings, auxiliary equipment, terminal markings, etc.
This article will focus on the fundamental information provided on the nameplate of a three-phase induction motor, since they are a commonly used industrial motor type. Most of this discussion can also apply to other motor types but would complicate the discussion, so they are omitted in order to ensure clarity. Fundamental information includes voltage, current, power, frequency, speed and service factor.
Does the motor use any of this information? No. The motor cannot read, right? The user of the motor can use this information to ensure safe, reliable performance from the motor. The user can also ignore some or all of this information and run the risk of subpar performance and possible serious consequences, such as shortened run life or reduced safety.
Voltage on the nameplate is generally the voltage intended to be applied at the terminals of the motor. It is not the voltage that is on the power grid. Generally, the standard motor nameplates are about 4% lower than the common grid rated voltage. The 4% lower voltage is because typical voltage losses in cable and other equipment supplying the motor can be substantial, lowering the power grid voltage at the motor terminals. Depending on the system, these voltage losses may affect the motor performance. Low voltage can cause lower torque, higher current, slower speed, higher motor temperature rise, etc. All of these impact motor run life and performance which is why the user should pay attention to the voltage at the motor terminals.
The focus has been on lower voltage so far because higher voltage is rarer and has less of a negative impact than low voltage. A slightly higher voltage may make the motor perform better. It just depends on the motor. However, an excessively higher voltage can also have detrimental effects that can cause overheating and affect insulation life. NEMA MG1 permits voltage variation of “plus or minus 10% of rated voltage with rated frequency. However, motor performance in this condition may not be in accordance with standards established for operation at rated voltage and frequency.”
The key takeaway with voltage is that significant variation of voltage from the nameplate will affect the motor performance and can impact the motor life. The motor will try to do what is asked of it, regardless of the nameplate voltage, by using the actual applied voltage.
The nameplate current is the current that is being used when the motor is being operated under a certain set of standardized conditions. The standardized conditions consist of the nameplate values for voltage, power output, speed and frequency. Many users assume that current is a direct indication of power output of the motor, but while they are related, they should not be equated.
While it can be a shortcut for evaluation of motor power output in certain conditions, current can be affected by a lot of factors besides the power output of the motor. The most obvious one is voltage,
but when talking about voltage, do not forget voltage balance. If the three voltage phases are unbalanced, higher currents will be required even if the average of the voltages matches the nameplate voltage. Lower voltage will typically cause higher currents for the same power output, so monitoring current does not directly reflect the power output.
Voltage with high harmonic content will also adversely affect current. In general, the current is a characteristic of how a user is operating the motor, so it can be useful to monitor current to assure the motor is being operated correctly. The motor will use as much current as is necessary to do the job under the conditions it is given.
Frequency is important as it affects not just the speed of the motor, but also output torque and ultimately the output power. The frequency on a nameplate is most commonly a single frequency of either 60 hertz (Hz) or 50 Hz, depending on the standard voltage supply frequency of the country.
However, many motors can be used at another frequency with some caveats. Talk to a motor supplier or manufacturer for other allowable frequencies, especially when using a variable speed drive (VSD).
Speed is generally a function of frequency and the motor design. The speed provided on the nameplate is what is expected under the specific conditions on the nameplate. Other speeds can occur and are dependent upon variables such as the actual load, voltage, frequency and other conditions.
If required operating speeds are substantially different than the rated speed, confirm with the manufacturer that the motor is capable of doing this. Some examples of critical characteristics that can be affected by changes in speed include motor cooling and bearing load capabilities. The motor will run at the speed it feels like unless controlled.
According to NEMA MG1, Part 1.42.,“Service factor of an alternating current (AC) motor is a multiplier which, when applied to the rated horsepower, indicates a permissible horsepower loading which may be carried out under the conditions specified for the service factor.” NEMA MG1 then refers to part 14.37, which goes into more detail; the most important of which states, “A motor operating continuously at any service factor greater than one will have a reduced life expectancy compared to operating at its rated nameplate horsepower. Insulation life and bearing life are reduced by the service factor load.” Keep in mind that service factor is mainly a North American thing. Most of the world does not use or even label motors with a service factor. Also, keep in mind that every motor supplier’s technical recommendations are to not intentionally
design the load to use the service factor. In other words, if it is a 100 hp motor with a 1.15 service factor, do not intentionally pick out a 110 hp pump. Service factor is intended to provide a margin of safety in case the load is higher than expected for short-term use. The motor will perform when operating within the power range defined by the service factor and even above—just not as normally expected.
The nameplate power is the rated output power. Always remember there is also input power that is a function of the efficiency of the motor.
Is the power listed on the nameplate the same amount of power being provided by the motor? Highly unlikely. Remember, the motor will try to do whatever is asked of it. The user asks the motor to do something by applying voltage, turning it on and applying a load on the shaft. What happens if the load on the shaft is exactly nameplate power output at nameplate voltage? Then the current and speed should be very near the nameplate values, assuming the voltage and frequency supplied is near the nameplate values. What happens if the load on the shaft is less than nameplate power? In general, the induction motor will turn ever so slightly faster, and the current will be reduced. The motor will likely run cooler and will not age as quickly as a fully loaded motor.
What happens if the load on the motor is higher but still within the service factor? As stated previously, the motor will continue to operate but may have a reduced life and will not perform quite as well. What happens if the load on the motor is higher than the service factor? Provided a user does not have controls to prevent this situation, the motor will try to provide that power even if it destroys itself in the process.
Remember the motor cannot read, so the nameplate horsepower will not be automatically followed. As the owner and operator, users have the means to ensure the motor does not destroy itself.
The motor nameplate is for the user to read, understand and then appropriately use the information in conjunction with the installation and operations manual. If the user installs good controls with settings based on the nameplate information, the motor will perform as expected and will have a long useful life.
If the nameplate is ignored, then there is a high likelihood the motor will fail to perform or have a shortened life. And we all want a long, happy life for man’s faithful servants, both dogs and motors.